1. Field of the Invention
The present invention relates to a remote-controlled circuit breaker and more particularly to a remote-controlled circuit breaker having an improved high-frequency making/breaking operation.
2. Description of the Related Art
FIG. 24 is a schematic diagram showing a conventional driving circuit for a three-phase induction motor M. An A.C. voltage is applied to the motor M through electric power lines (not shown) via a conventional circuit breaker 1 and a magnetic contactor 2 connected in series with each other. The circuit breaker 1 is provided primarily to protect the motor M and connection wires 4 from heat damage caused by a short-circuit or an overload. Since the rated lifetime of the contacts of a conventional circuit breaker is generally under 10,000 switching cycles, the circuit breaker 1 is not suitable for making/breaking its contacts with high frequency. Further, it is difficult to effect remote-control operation of a conventional circuit breaker such as the circuit breaker 1.
On the other hand, the magnetic contactor 2 is suitable for making/breaking its contacts with fairly high frequency. If only the magnetic contactor 2 were used to drive the motor M, however, without the series-connected circuit breaker 1, welding of contacts in the magnetic contactor 2 could occur when a large current flows through the contacts as a result, for example, of a short circuit, rendering the magnetic contactor useless. For the above-mentioned reasons, the circuit breaker 1 and the magnetic contactor 2 are connected in series with each other, thereby realizing both a breaking function in response to an excessive current and a high-frequency making/breaking function susceptible to remote control.
As shown in FIG. 25, both the circuit breaker 1 and the magnetic contactor 2 are conventionally fixed to a common casing 3 to constitute a protection and control unit.
However, since the circuit breaker 1 and the magnetic contactor 2 are separate devices, many interconnecting wires 4 are necessary in the casing 3. In order to provide sufficient space to accommodate both devices (the circuit breaker 1 and the magnetic contactor 2), the interconnecting wires 4, and various wire connecting devices, the casing 3 must unavoidably be large.
U.S. Pat. No. 4,631,507 discloses a switching device having contacts which are actuated either by an armature of a remote-controllable electromagnet or by a tripping device. However, the mechanism for transmitting motion of the armature is provided independently of that for transmitting motion of the tripping device to the contacts without any substantial common component. Therefore, construction of the switching device is complicated, and the switching device is still not as compact as possible.